Chapter 22 – The Light Reactions (Problems 1,2,4,5,8-10,13-19)

• Photosynthesis: a process that converts atmospheric CO2 and H2O to carbohydrates

• Solar energy is captured in chemical form as ATP and NADPH

• ATP and NADPH are used to convert CO2 to hexose phosphates

• Phototrophs: photosynthetic organisms (some bacteria, algae, higher plants)

Energy + 6H2O + 6CO2 C6H12O6 + O2

C6H12O6 + O2 6H2O + 6CO2 + Energy

Photosynthesis

Light

ETC/OX-Phos

ATPGlycolysis/CAC

Dow

nhill E

nerg

y Pr

oces

s

Uph

ill E

nerg

y Pr

oces

s

Respiratory ETC Photosynthesis ETC

Energy + 6H2O + 6CO2 C6H12O6 + O2

C6H12O6 + O2 6H2O + 6CO2 + Energy

Photosynthesis

Light

ETC/OX-Phos

ATPGlycolysis/CAC

Light and dark reactions

• Both processes can occur simultaneouslyReactions that require light (light reactions):

H2O + ADP + Pi + NADP+ O2 + ATP + NADPH + H+

Reactions which do not require light (dark reactions):

CO2 + ATP + NADPH + H+ (CH2O) + ADP + Pi + NADP+

Sum: CO2 + H2O (CH2O) + O2

Three membranes, and three “spaces”.

CO2 + ATP + NADPH + H+ → (CH2O) + ADP + Pi + NADP+

H2O + ADP + Pi + NADP+ → O2 + ATP + NADPH + H+

22.1 Photosynthesis Takes Place in Chloroplasts

22.2 Photosynthesis Transforms Light Energy into Chemical Energy

I II I II I II I II

Structures of Chlorophyll and bacteriochlorophyll

• Chlorophylls - usually most abundant and most important pigments in light harvesting

• Contain tetrapyrrole ring (chlorin) similar to heme, but contains Mg2+

• Chlorophylls a (Chl a) and b (Chl b) in plants

• Bacteriochlorophylls a (BChla) and b (BChlb) are major pigments in bacteria

Light harvesting complexes enhance the efficiency of photosynthesis

Accessory pigments

Reaction centers of the photosystems

• PSI and PSII each contain a reaction center (site of the photochemical reaction)

• Special pair: two chlorophylls in each reaction center that are energized by light

• In PSI special pair is: P700(absorb light maximally at 700nm)

• In PSII the special pair is: P680 (absorb light maximally at 680nm)

Some Plants Produce Toxins; e.g. Potatoes

22.3 Two Photosystems Generate a Proton Gradient and NADPH

Photosystem I: Ferredoxin and NADPH Production

H

+

NADP+ NADPH

Problem: Fd accepts and donates electrons one at a time, but NADP+ accepts a pair of electrons.

Solution: Fd NADP+

reductase.

Fd NADP+ reductase uses the co-factor FAD which can accept electrons one at a time from Fd and donate a pair of electrons to NADP+ to form NADPH

P700

P700*

P700+

From PC

FdNADP+ + H+

Fd-NADP+ oxidoreductase(via FAD/FADH2—why?)

NADPH

Photosystem I

Reduction of NADP+ (Eo’ = -0.32 V) by Fd (Eo’= -0.43 V) is catalyzed by ferredoxin-NADP+ oxidoreductase.

Photosystem II: Transfers Electrons to PSI and Generates Proton Gradient

PSII: Reduction, excitation and oxidation of P680

• P680 special-pair pigment of PSII

• P680+ is reduced by e- derived from oxidation of H2O

• Light energizes to P680*, increasing its reducing power

Pha

x 2

2H+ + e-e- PAQ

PAQH2(bound)

PBQH2(soluble)

2H2O → O2 + 4H+ + 4e-

Via OEC

Cytbf

Transfer of electrons from QH2 to plastocyanin

QH2 + 2Pc(Cu2+) →    Q + 2 Pc(Cu+) + 2H+Cytbf

Thylakoid lumen

PSI

Photosystem II: Electrons are Transferred from Q to Pc via Cytbf, and then from Pc

to PSI

2H2O → O2 + 4H+ + 4e-

Via OEC

Photosystem II: The Evolution of O2; the

OEC of PSII

2H2O → 4e- + 4H+ + O2

The manganese center of PSII

D1-Tyr-OH → D1-Tyr-O. + e-D1-Tyr-O. + e- → D1-Tyr-OH

To P680+

From Mn

Cooperation between PSI and PSII

22.4 A Proton Gradient Drives ATP Synthesis

2H2O + 2NADP+ + 10H+stroma → O2 + 2NADPH + 12H+

lumen3ADP3- + 3Pi2- + 3H+ + 12H+

lumen → 3ATP4- + 3H2O + H+stroma

2NADP+ + 3ADP3- + 3Pi2- + H+ → O2 + 2NADPH + 3ATP4- + H2O

Cyclic Electron Flow

When NADPH/NADP+ ratio is high …..

Thylakoid lumen

Stroma

Organization of Photosynthesis Components

Stroma

Thylakoid lumen

Organization optimizes locationPSI for NADPH production instroma, PSII for increasingproton concentration inthylakoid lumen, and ATPsynthase for production ofATP in stroma

HerbicidesPSII (QH2Inhibition)

PSI e acceptor